Particles, anisodimensional

Several colloidal systems containing anisodimensional particles, such as iron oxide and tobacco mosaic virus sols, show reversible... 

That the phenomena mentioned in this section are exclusively found with anisodimensional particles should be explain by the relatively enormous flat (or cylindrical) surface of these particles. When orientated parallel, cylindrical particles may be on the favourable distance of each other over their whole length, diskshaped particles even in two dimensions. Moreover with cylindrical particles a configuration can be formed in which each particle is surrounded by six other ones, thereby enhancing the interaction six-fold... 

The phenomenon becomes more complicated if deformation also occurs as well as. orientation The theory has been developed so far under the assumption that the particles do not influence each other thus for dilute sols. That this is not quite correct appears from the fact that the specific double refraction still shows a dependence on the concentration. In Chapter IX, 5d, p. 363 on thixotropy it will be seen that it is just in sols with anisodimensional particles that a special tendency to orientation occurs. This cooperates with the flow orientation so that the effect is strengthened ... 

Several colloidal systems containing anisodimensional particles may under certain circumstances show a reversible separation into two phases. The oldest example is the iron hydroxide sol described by Cotton and Mouton The same phenomenon has been studied more extensively on iron hydroxide, vanadium pentoxide, tungsten trioxi-de, benzopurpurin and other dyes by Zocher % Szegvari Heller and recently on tobacco mosaic virus by BerNal and Fankuchen and cn cucumber virus by Bawden and Pterie ... 

ANOMALOUS TYPES OF FLOCCULATION a Gel formation in systems with anisodimensional particles... 

With anisodimensional molecules states have indeed been observed which lie between the crystalline and the amorphous (mesophases, liquid crystals). Mark recognizes the following transitions between solid crystals and amorphous liquids 1°. Three-dimensional crystal the centres of gravity of the units are fixed (apart from vibration), rotations are not possible. Examples hexamethylene tetramine, urea. 2°, Crystal with rotating molecules the centres of the particles are fixed rotation about one or more axes is possible. Examples NH4CI, sodium stearate at higher temperatures. 

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